As an emission type electronic displaying device, there is an electroluminescent device (ELD). As elements constituting the ELD, there is an inorganic electroluminescent element or an organic electroluminescent element (hereinafter referred to also as organic EL element). The inorganic electroluminescent element has been used for a surface illuminant, however a high voltage alternating electric power has been required to drive the element.
An organic EL element has a structure in which a light emission layer containing a light emission compound is provides between a cathode and an anode, and an electron and a hole are injected into the light emission layer and recombined to form an exciton. The element emits light utilizing light (fluorescent light or phosphorescent light) generated by inactivation of the exciton, and the element can emit light only by applying a relatively low voltage of from several volts to several decade volts. A display using an organic EL element has a wide viewing angle and a high visuality since the element is of self light emission type. Further, the element is a thin, complete solid element, and therefore, the element has attracted attention from the viewpoint of space saving and portability.
For practical use in the near future, an organic EL element exhibiting high emission intensity, high efficiency and low power consumption is required. For example, the following methods are disclosed in the prior art: (i) an EL element using stilbene derivatives, distyrylarylene derivatives or tristyrylarylene derivatives, doped with a slight amount of a fluorescent compound, to increase emission intensity and to attain long life of the element (for example, Patent Document 1); (ii) an EL element which comprises an organic light emission layer containing an 8-hydroxyquinoline aluminum complex as a host compound doped with a slight amount of a fluorescent compound (for example, Patent Document 2); and (iii) an EL element which comprises an organic light emission layer containing an 8-hydroxyquinoline aluminum complex as a host compound doped with a quinacridone type dye (for example, Patent Document 3).
When light emitted through excited singlet state is used in the element disclosed in the above Patent documents, the upper limit of the external quantum efficiency (ηext) is considered to be at most 5%, since the generation ratio of singlet excited species to triplet excited species is 1 to 3, which results in that the generation probability of excited species capable of emitting light is 25%, and further, external light emission efficiency is 20%.
Since an organic EL element, employing phosphorescence through the excited triplet, was reported by Princeton University (for example, see M. A. Baldo et al., Nature, 395, p. 151-154 (1998)), study on materials emitting phosphorescence at room temperature has been actively made (for example, see Non-Patent Document 2 or Patent Document 4).
As the upper limit of the internal quantum efficiency of the excited triplet is 100%, the light emission efficiency of the exited triplet is theoretically four times that of the excited singlet. Accordingly, light emission employing the excited triplet exhibits almost the same performance as a cold cathode tube, and can be applied to illumination.
In order to improve the emission intensity and the emission life of an organic EL element, formation of a positive hole (hereinafter also referred to as a hole) blocking layer between an cathode and a light emission layer has been proposed. The formation of a hole blocking layer helps accumulating holes in a light emission layer resulting in improving a recombination rate of a hole and an electron which also results in attaining a higher emission efficiency. It has been reported that a single use of a phenanthroline derivative or a triazole derivative is effective as a hole blocking material (for example, Patent Documents 5 and 6). Also, a long life organic EL element have been prepared by using a certain specified aluminum complex as a hole blocking layer (for example, Patent Document 7).
By introducing a hole blocking layer in an green organic EL element using a phosphorescent material, an internal quantum efficiency of almost 100% and a life of 20,000 hours have been attained (for example, Non-Patent Document 3), however, problems still remain in view of emission intensity.
In a case when a blue—blue green phosphorescent compound is used as a dopant, an organic EL element in which a carbazole derivative like CBP is used as a host material have been reported (non-Patent Document 4), however, the external quantum efficiency is around 6%, which is not fully satisfactory, so that an improvement is required. As for a blue organic EL element in which an emission of a fluorescent compound is used and a connecting group is introduced in a biaryl site of a carbazole derivative existing in a center of the molecule, an organic EL element being excellent in color purity and exhibiting a long life has been prepared (for example, Patent Document 8). A longer life of an organic EL element has been attained when a certain specified five-coordinate metal complex is used in a hole blocking layer in addition to the above mentioned compound and a phosphorescent compound is used as a dopant (for example, Patent Document 9).
Other organic EL elements using a carbazole derivative have been proposed (for example Patent Documents 10-21), however, the carbazole derivatives disclosed in the above patent documents have not been fully satisfactory in giving an organic EL element exhibiting an emission efficiency and a heat resistance suitable for practical use. Development of an organic EL element giving a higher emission intensity with a lower electric power consumption and giving a longer life is desired.
(Patent Documents 1)                Japanese Pat. No. 3093796        
(Patent Documents 2)                Japanese Patent Publication Open to Public Inspection (hereinafter referred to as JP-A) No. 63-264692        
(Patent Documents 3)                JP-A No. 3-255190        
(Patent Documents 4)                U.S. Pat. No. 6,097,147        
(Patent Documents 5)                JP-A No. 8-109373        
(Patent Documents 6)                JP-A No. 10-233284        
(Patent Documents 7)                JP-A No. 2001-284056        
(Patent Documents 8)                JP-A No. 2000-21572        
(Patent Documents 9)                JP-A No. 2002-8860        
(Patent Documents 10)                JP-A No. 2002-203663        
(Patent Documents 11)                JP-A No. 8-3547        
(Patent Documents 12)                JP-A No. 8-143861        
(Patent Documents 13)                JP-A No. 8-143862        
(Patent Documents 14)                JP-A No. 9-249876        
(Patent Documents 15)                JP-A No. 11-144866        
(Patent Documents 16)                JP-A No. 11-144867        
(Patent Documents 17)                JP-A No. 8-60144        
(Patent Documents 18)                JP-A No. 2002-8860        
(Patent Documents 19)                JP-A No. 2003-77674        
(Patent Documents 20)                WO No. 03/50201        
(Patent Documents 21)                JP-A No. 2003-231692        
(Non-Patent Documents 1)                M. A. Baldo et al., Nature, 395, 151-154 (1998)        
(Non-Patent Documents 2)                M. A. Baldo et al., Nature, 403(17), 750-753 (2000)        
(Non-Patent Documents 3)                The 62nd Japan Society of Applied Physics academic Meeting, draft 12-a-M7; and Pioneer technical-information magazine, Vol. 11, No. 1        
(Non-Patent Documents 4)                The 62nd Japan Society of Applied Physics academic Meeting, draft 12-a-M8        